1. Field of the Invention
The present invention relates to a developing material for electrostatic images, more particularly to a developing material for electrostatic images for use in a laser printer.
2. Description of the Prior Art
A known developing material for electrostatic images comprises a mixture of smaller resinous toner particles and larger carrier particles made of iron beads. The toner particles are held on the surface of the carrier particles by electrostatic force, which develops from the triboelectric charging of the toner particles and the carrier particles in opposite polarities due to contact therebetween. When the developing material is moved into contact with the latent electrostatic images formed on a photoreceptor, the toner particles are attracted to the latent images, and, thus, the images are developed. The developed images are transferred onto plain paper and fixed thereon by heating.
Electrostatic developing materials are used in copying printers, printers for computer systems, and the like. They allow dry, high speed, high resolution image printing on diverse kinds of paper.
The demand for electrostatic developing material has particularly grown in the field of laser printers for computer systems. Recent printers for computer systems must be able to output at a high resolution not only the relatively simple alphanumeric type but also very complicated Chinese characters. Laser printers are advantageous in that a laser beam usually has a spot diameter of from 100 to 150 .mu.m.
Generally, the bulk electric resistivity (below, "electric resistivity") of the carrier particles is in linear proportion with the toner parameter, "toner parameter" being defined as the amount of positive charge (.mu.C) on the toner particles per unit weight (g) of toner particles.
A desirable toner parameter is considered to be from 10 to 30 .mu.C/g, depending on the type of the development system, which corresponds to an electric resistivity of the carrier particles of from 10.sup.5 to 10.sup.7 .OMEGA..multidot.cm.
If the electric resistivity of the carrier particles is too high, the negative charge on the carrier particles increases until almost the saturation point (below, "charge-up of carrier particles"), making it difficult to generate a further negative charge on the carrier particles and, consequently, to impart a counter positive charge on the toner particles, as required for effective functioning of the developing material.
In addition, a high electric resistivity of the carrier particles leads to an increased potential drop through the so-called "magnetic brush" on the magnetic drum of the developer, preventing sufficient toner particles from depositing on the latent images.
To avoid these problems, the electric resistivity of the carrier particles must be kept low. A conceivable method of doing this would be to coat the carrier particles with a resin containing carbon powder. It is difficult, however, to keep the carbon powder uniformly dispersed in the carrier coating resin, because the powder tends to separate from the coating resin while the coated carrier particles are being mixed in the developer.
Another problem is "toner filming". Toner filming occurs due to a number of collisions between the toner particles and carrier particles. The attendant mechanical friction causes the toner material to partially melt and fuse on not only the surface of the carrier particles, but also the surface of the photoconductive drum, i.e., photoreceptor. Such fused toner is called "spent toner". Toner filming makes the surface of the carrier particles substantially the same as the toner particles and thereby deteriorates in the triboelectric property. It also makes the surface of the photoconductive drum less optically sensitive and, so, produces afterimages.
To prevent toner filming on carrier particles, it is known to provide the carrier beads coated with a fluorocarbon layer. Such a fluorocarbon layer, however, exhibits too high an electric resistivity and poor mechanical strength.
Proposals have also been made to solve this problem by using a filming-resistant material as the binder resin of the toner particles, but these materials usually have had high melting points, which have resulted in incomplete thermal fixation.